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Full text of "Text Book Of Mechanical Engineering"

612

Reversed Action and Second Law.

zero/ a condition practically unattainable, and all the heat in the
working substance can never be utilised. The energy obtainable is
only that between the available temperatures, and the difference of
T-J and r2 should therefore be as large as is practically, possible.

Reversed Action occurs, as previously suggested, when
expansion takes place along i 4, 4 3, and compression along 3 2,
2 i, the operations being entirely the reverse of those just con-
sidered. External work is done on instead of by the gas, and
heat is taken from the cold body and rejected into the hot body. No
better practical example of a reversed cycle can be given than
that of an air-compressing engine as at Fig. 562, p. 546.

Let it be possible to have an engine (No. 2) of equal power
but higher efficiency than Carnot's (No. i); and let No. 2 drive
No. i in reverse order. Then No. 2, taking its heat from the hot
body and rejecting into the cold body, and giving all its externaf
work towards driving No. i, the latter is thus made to take heat
from the cold body, which, together with the work received, it
delivers into the hot body. No external work being left over, the
contrivance is self-acting.

;. Let H2 be the*heat taken from the hot body by No. 2, and h^
that rejected into the cold body; Hj the heat rejected into the
hot body by No. i, and h^ that taken from the cold body. Power
being equal,

(Reversed) H!-A! = H2-/&2 (Direct) . . . (a)

Efficiency of No. i = 5tZ*i       Efficiency of No. 2 - ^^

H

But

is to be greater than ^
6                    %

, by (a), the numerators are equal,
H2 must be less than Hx.

(See pp. 770, 773,
883, and 1132.)

The heat taken from is therefore less than that given to the
hot body, and by a self-acting process heat is being taken from the
cold and delivered to the hot body, which is impossible by the

Second Law of Thermodynamics.—Heat cannot pass
from a cold to a hot body without external aid* This is the
resojt of experience, the tendency being always to equalisation